Let's write down some variables:
v: speed of sound; v = 340 m/s
f: frequency of wave; f = 20000 Hz
λ: wavelength; this is what we're trying to solve for
Given these variables, we can use the following equation to solve for λ:
v = fλ
Now let's plug in our known variables and solve for λ:
340 = 20000λ
λ = 340/20000
λ = 0.017 m